The present invention is related to hydro-pneumatic type brake actuators and more particularly to brake actuators of the above type, which are suitable for use in freight-type railway brake systems.
At the present time, railroads typically employ traditional automatic air brake systems. Each car in a train is normally equipped with auxiliary and emergency reservoirs which are charged from a brake pipe extending through the train, and a control valve which responds to changes in the brake pipe pressure to control the flow of air to and from the car brake cylinders. Since the railroad industry has standardized on relatively low braking pressures, and practical considerations limit the diameter of the car brake cylinders, it has become necessary to employ force-multiplying linkages between the brake cylinder and brake shoes in order to obtain the high braking forces required at the brake shoes. Such a brake rigging arrangement inherently increases the stroke of the brake cylinder piston required to move the brake shoes enough to take up the clearance space between the brake shoes and wheel treads. Accordingly, the brake cylinder clearance volume or, in other words, the piston stroke required to bring the shoes into wheel contact, must be relatively large and thus requires a considerable amount of air. This, in turn, requires relatively large air reservoirs, which are space-consuming and thus impose a further restriction on the area needed for the force-multiplying brake rigging.
The gradual acceptance of hydraulic brake systems in the rapid transit segment of the railway industry suggests the possibility of using hydraulics as a means of transmitting brake forces to the brake shoes in freight-type service. Such an approach would be advantageous in eliminating the need for the cumbersome, mechanical brake rigging presently employed on freight cars to transmit the brake cylinder forces to the brake shoes. Ideally, such an arrangement would require only a single hydro-pneumatic actuator device on each car corresponding to the brake cylinder in a conventional mechanical brake rigging system. Mechanical advantage sufficient to obtain the desired high brake shoe forces would be obtained by a large ratio piston of the hydraulic actuator. Because of this high ratio piston and the brake shoe clearance to be taken up, the stroke of the actuator piston in the hydraulic actuator would be necessarily large, and thus require a considerable amount of air simply to bring the brake shoes into braking engagement with the wheel treads. This would unduly enlarge the size of the hydraulic actuator, as well as require relatively large air reservoirs.